The invention relates to an optical information medium for recording by means of a focused radiation beam having a radiation wavelength λ and a numerical aperture NA, said medium having a substrate, a stack of layers provided thereon, the stack comprising at least a first recording stack and k radiation beam transmissive layers, each radiation beam transmissive layer having a refractive index ni and an average thickness di μm and 1≦i≦k and k≧2.
The invention further relates to a method of manufacturing such an optical information medium.
An embodiment of such an optical recording medium is known from European patent application EP-A-1047055. In particular the application of a light transmissive adhesive layer in order to bond cover layers or other layers to each other, to the surface of a substrate and/or to one or more information storage layers is described.
There is a constant drive for obtaining optical storage media suitable for recording and reproducing, which have a storage capacity of 8 Gigabyte (GB) or larger. This requirement is met by some Digital Video Disc or sometimes also Digital Versatile Disc formats (DVD). DVD formats can be divided into DVD-ROM that is exclusively for reproduction, DVD-RAM, DVD−RW and DVD+RW, which are also usable for rewritable data storage, and DVD-R, which is recordable once. Presently the DVD formats comprise discs with capacities of 4.7 GB, 8.5 GB, 9.4 GB and 17 GB.
The 8.5 GB and, in particular, the 9.4 GB and 17 GB formats exhibit more complicated constructions and usually comprise multiple information storage layers. The 4.7 GB single layer re-writable DVD format is easy to handle comparable, for example, to a conventional CD (compact disc) but offers an insufficient storage capacity for video recording purposes.
A high storage capacity format that recently has been suggested is Digital Video Recordable disc (DVR). Two formats are currently being developed: DVR-red and DVR-blue, where red and blue refer to the used radiation beam wavelength for recording and reading. This disc overcomes the capacity problem and, in its simplest form, has a single storage layer format which is suitable for high density digital video recording and storage having a capacity up to 22 GB in the DVR-blue format.
The DVR disc generally comprises a disc-shaped substrate exhibiting on one or both surfaces an information storage layer. The DVR disc further comprises one or more radiation beam transmissive layers. These layers are transmissive to the radiation beam that is used to read from or write into the disc. For example a transmissive cover layer, which is applied on the information storage layer. Generally, for high-density discs, lenses with high numerical aperture (NA), e.g. higher than 0.60, are used for focusing such a radiation beam with a relatively low wavelength. For systems with NA's above 0.60 it becomes increasingly difficult to apply substrate incident recording with substrate thicknesses in the 0.6–1.2 mm range due to decreasing tolerances on e.g. thickness variations and disc tilt. For this reason, when using discs that are recorded and read out with a high NA, focusing onto a recording layer of a first recording stack, is performed from the side opposite from the substrate. Because the first recording layer has to be protected from the environment at least one relatively thin radiation beam transmissive cover layer, e.g. thinner than 0.5 mm, is used through which the radiation beam is focused. Clearly the need for the substrate to be radiation beam transmissive no longer exists and other substrate materials, e.g. metals or alloys thereof, may be used.
In case second or further recording stacks are present a radiation beam transmissive spacer layer is required between the recording stacks. The second and further recording stacks must be at least partially transparent to the radiation beam wavelength in order to making writing in and reading from the recording layer of the first recording stack possible. The thickness of such spacer layers typically is from the order of tens of micrometers. The radiation beam transmissive layer or layers which are present between the radiation beam source and the recording stack that is most remote from the substrate are normally called cover layers. When prefabricated sheets are used as transmissive layers extra transmissive adhesive layers are required in order to bond cover layers to each other.
In the DVR disc the variation or unevenness of the thickness of the radiation beam transmissive layers over the radial extension of the disc has to be controlled very carefully in order to minimize the variation in the optical path length for the impinging radiation. Especially the optical quality of the radiation beam at the focal point in the DVR-blue version, which uses a radiation beam with a wavelength substantially equal to 405 nm and an NA substantially equal to 0.85, is relatively sensitive to variations in the thickness of the transmissive layers. The total layer thickness has an optimal value in order to obtain minimum optical spherical aberration of the focused radiation beam on, e.g., the first information recording layer. A slight deviation, e.g. +/−2 μm, from this optimal thickness already introduces a considerable amount of this kind of aberration. Because of this small range it is important that the average thickness of the transmissive layers is equal to or close to its optimal thickness in order to make optimal use of the tolerances of the system and to have a high yield in manufacturing the disc. Assuming that a thickness error is Gaussian distributed around the nominal setting of the thickness, it is clear that the number of manufactured discs which do not comply with the above specification is minimal when the target setting of the nominal thickness during manufacture is substantially equal to the optimal thickness of the cover layer as in the specification of the DVR disc. The nominal thickness of a single layer cover of the DVR disc is 100 μm when the refractive index of the cover layer is n=1.6. The nominal thickness of the cover layer has to be adjusted when using a different refractive index. Since a change in optimal thickness can exceed more than one micron, it is clear from the point of view of yield that even this small change has to be taken into account. Because of the high numerical aperture of the read- and write system such a change in optimal cover layer thickness, when the refractive index is different, can not accurately be predicted using e.g. third order Seidel aberration analysis. Therefore higher order analysis or ray tracing methods have to be used. Let D(n) be defined as the optimal thickness of the single cover layer as a function of the refractive index, hence, for the proposed thickness, D(1.6)=100 μm. Since this is a one parameter function, it has to be calculated once, and can be presented in a single graph. A problem now arises when considering multi transparent layer discs. As described earlier multi-layer discs are used to allow for e.g. dual-layer readout. Furthermore, from EP-A-1047055 it is known to use a polymer layer such as, for example, a polycarbonate (PC) sheet as light-transmissive cover layer and adhere such layer to the information storage layer by means of a thin, spin-coated layer of a UV curable liquid resin or a pressure sensitive adhesive (PSA). Because the disc now is built up of more than one radiation beam transmissive layer it becomes even more difficult to manufacture the disc which varies within the above specified range. Hence for such a disc it is even more important to set the nominal thicknesses substantially equal to the optimal nominal thicknesses of the multiple cover layer of the disc. Because this is now a multi-variable function it can not be presented in a few simple graphs. A way to solve this problem is using ray-trace methods. The problem is now that every manufacturer of optical discs, who applies transparent layers with deviating refractive indices, must calculate the optimal thickness itself since it is not known beforehand. An essential element in the ray-trace formalism is that the designer has to define the correct merit function that the ray trace program needs in order to correctly optimize one or more transparent layers of the disc. This requires a skilled optical designer, and the above way is susceptible for errors.